Thermochemical equations• A typical chemical equation is S + O2 SO2

• It is called a “thermochemical equation” when we add information about H …

S + O2 SO2 H = – 296.9 kJ• If we change the equation, then the H also

changes … SO2 S + O2 H = + 296.9 kJ

• If the reaction is reversed the sign is reversed• Also, if numbers in the equation change, so will

the amount of energy produced/absorbed:2S + 2O2 2SO2 H = – 593.8 kJ

• Read 1st part of handout. Do Q 1-3

Answers

C + O2

CO2393.5 kJ

Q2 H2 + I2 2HI H = + 53.2 kJ

Q1 C + O2 CO2 H = – 393.5 kJ

Q3 1/2H2 + 1/2I2 HI H = + 26.6 kJ

2HI

53.2 kJH2 + I2

Answers pg. 175, Q 5.45-5.50

5.45 - At STP (25°C and 1 atm)

5.46 - The H° value

5.47 - Moles. (You can’t have 1/2 an atom)

5.48 - answer in back of book

4Al(s) + Fe2O3(s) 2Al2O3(s) + 4Fe(s)

H° = -1708 kJ

Answers pg. 175, Q 5.45-5.505.49 - If 2 = 6542 kJ then 1.5 = 4907 kJ

(6542/2 x 1.5)

Actually, it should be -4907 kJ since it is exothermic

5.50 -

10CaO(s) + 10H2O(l) 10Ca(OH)2(s)

H° = -653 kJ

Equation is reversed, thus H° sign changes

Equation is multiplied by 10, thus so is H°

Standard heats of reaction5.45 ° indicates 1 atm (and is usually

associated with a temperature of 25°C)5.46 An energy term is added (e.g. H or H°)5.47 Moles5.48Al(s) + 1/2Fe2O3 (s)Fe(s) + 1/2Al2O3 (s) H°= – 426.9 kJ4Al(s) + 2Fe2O3(s) 4Fe(s) + 2Al2O3 (s) H° = – 1708 kJ5.49 1.5 is 3/4 of 2. 3/4 of 6542 kJ is 4906.5 kJ5.50 10CaO(s) + 10H2O(l) 10Ca(OH)2(s) H°=–653kJ

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